conf: Use nodes in virDomainLoaderDefParseXMLNvram()
conf: Use nodes in virDomainLoaderDefParseXMLNvram()

This makes the function more consistent with
virDomainLoaderDefParseXML() by preferring the virXMLProp
class of functions to XPath access.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt